1. Field of the Invention
The present invention relates to fume hood enclosures used for worker protection. More particularly, the present invention relates to a method and apparatus for stabilizing the vortex in both existing and new fume hoods.
2. Description of Related Art
The Occupational Safety and Health Administration (OSHA) defines a fume hood as a four sided exhausted enclosure with a front opening for worker arm penetration. OSHA defines a safe fume hood where worker exposure levels are below the permissible exposure limits (PELs) accepted by government and private occupational health research agencies, including the National Institute of Occupational Safety and Health (NIOSH). OSHA's position is that it is an employer's responsibility to make hood adjustments or replace hoods as necessary when an employer discovers, through routine exposure monitoring and/or employee feedback, that the fume hoods are not effectively reducing employee exposures.
OSHA no longer recommends a given face velocity in feet per minute (fpm) as a reference to worker protection. This is a reversal of OSHA's early 1980's face velocity position when 125 to 150 fpm was recommended for extreme toxic material, 100 to 125 fpm for most materials and 75 to 100 fpm for nuisance materials, dust, and odors. OSHA's earlier position on face velocity and a fume hood's capture protection theory prompted the development of methods to vary exhaust airflow volume of a fume hood in response to varying sash opening positions as a way to maintain a fixed face velocity in fpm.
This type of fume hood, often referred to as a variable air volume (VAV) fume hood, had the potential to save energy associated by reducing the amount of conditioned make-up air exhausted, and therefore reducing the amount of conditioned make-up air wasted. For example, at $0.10 per kilowatt-hour, and depending on hood geographical location, it costs approximately $3.50 to $6.50 a year in the United States to replenish one cubic foot per minute (cfm) of conditioned make-up air exhausted by the fume hood. An average prior art constant air volume six foot fume hood will consume over $300,000 in electrical energy over its expected lifetime. U.S. Pat. No. 4,741,257 pioneered closed-loop variable air volume fume hood control and U.S. Pat. Nos. 4,528,898; 4,705,553; 4,773,311; and 5,240,455 proposed open-loop variable air volume fume hood control. VAV fume hood technology dominated how fume hoods were operated through the 1980's and early 1990's.
Fume hood performance testing prior to OSHA's 1990 Laboratory Worker Regulation was based on smoke visualization and face velocity measurement. Smoke bombs or sticks were placed within the fume hood's enclosure, and as long as the smoke was not seen exiting the fume hood, it was deemed safe to use at the design face velocity. In the early 1990's, a standardized performance tracer gas analysis test began to be used to quantitatively measure fume hood performance in actual spillage rates in parts per million (ppm). The results have a relationship to PELs as determined by NIOSH. The tracer gas testing was developed to address medical studies linking increased birth defects and cancer rates among laboratory workers as highlighted in OSHA's Jan. 31, 1990final rule, 29 CFR Part 1910, on Occupational Exposures to Hazardous Chemicals in Laboratories. The tracer gas test takes into account the influence of a worker in front of the fume hood and analyzer sampling rate set to replicate the average worker breathing.
NIOSH fume hood tracer gas cited published studies indicate variable air volume and constant volume controlled fume hoods did maintain face velocity and may have saved energy but did little to improve worker safety. The tests revealed fume hood designs based on vapor capture face velocity theory failed to work as well, and protect workers from spillage, as manufacturers had suggested.
NIOSH, whose mission is to provide national and world leadership to prevent work-related illness and injury, published a position paper in 2000 stating that fume hood face velocity is not an adequate predictor of fume hood spillage. Additionally, tracer gas fume hood studies indicated between 28% and 38% of the existing stockpile of 1,300,000 to 1,400,000 hoods in the United States fail to meet minimum worker protection, even after attempts to adjust the fume hoods to improve performance. At that time, NIOSH's fume hood failure statistics were based on the American Industrial Hygiene Association's acceptable average fume hood tracer gas spillage rate of 0.1 ppm. In 2003, the acceptable tracer gas spillage rate was reduced by half to a rate 0.05 ppm. As a result, NIOSH's earlier estimates of unsafe fume hoods have nearly doubled.
The fume hood manufacturer's own trade organization, Scientific Equipment Furniture Association (SEFA) went on record in their SEFA 1-2001 “Laboratory Fume Hoods Recommended Practices” indicating, “Face velocity shall be adequate to provide containment. Face velocity is not a measure of safety.” This was the first time the fume hood manufactures abandon the face velocity capture theory. The SEFA 1-2000 also stated that the “acceptable 0.05 ppm tracer gas spillage level shall not be implied that this exposure level is safe.”
In terms of fume hood design, the problem was further compounded by the fact that prior art fume hoods were designed and specified by architects as furniture, as opposed to being designed, tested and specified by engineers as mechanical equipment. The early day fume hoods used stack height and candles placed on the fireplace smoke shelf to create draft. In the 1800's gas rings replaced candles and eventually fans and electric motors replaced gas rings. Changes, such as adding a front vertical single sash window instead of a hinged door, were eventually instituted. Prior art vertical or combination sash hoods all incorporate a counter balance weight system. Over time, these counterbalancing sash weight systems fail or become difficult to move. Repairing the counter balance weight systems require the fume hood be removed, which requires disconnecting all electrical, plumbing and exhaust services. As this puts the hood out of service for a period of time, the sash maintenance is rarely done. Instead, when the sash is no longer moveable it is blocked open with the counter weight balancing system abandoned in place.
In the 1940's a back exhaust baffle system and streamlined shape “picture window” entrance and work surface airfoil were introduced to all hoods, as illustrated in FIG. 1. Early prior art fume by-pass hood 10 has a vertical moveable sash 18 and a picture window utility post 17. There is a rear baffle conduit 28 with a manually adjusted lower slot 36, a fixed center slot 34, and manually adjusted upper or top slot 32. An exhaust duct 38 is shown on top of the hood and a work surface airfoil 22. Because prior art fume hoods only considered face velocity, no thought was given to the uneven back baffle 28 energy distribution caused by the very narrow but wide plenum design, and its negative effect on internal airflow patterns. The sole purpose for the back baffle was to create a flat face velocity, which was subsequently found to be an ineffectual design premise. Prior art fume hood picture window design posts, utility water and gas handle silhouettes and vertical and or horizontal sash guide channels, all contributed to cause localized eddies and airflow reversals to form at the utility post openings. In the 1950's, an air bypass diffuser 31 was added above the sash opening in an attempt to produce uniform face velocity with sash closure.
To save energy in the 1960's, un-conditioned auxiliary make-up air was introduced above and around the sash perimeter. U.S. Pat. Nos. 3,025,780; 3,111,077; 3,218,953; 3,254,588; 4,177,717; 4,436,022 and 6,080,058 describe various methods used in introducing un-conditioned outside auxiliary make up air into a fume hood. One example of an auxiliary make up fume hood design is shown in FIG. 2. The outside air supply duct 39 is attached to the full width supply plenum 40. There is a vertical full width perforated distribution diffuser 41 in the supply plenum 40 along with air turning vanes 42. The supply velocity into the supply slot is 250-300 fpm. The maximum auxiliary air supply volume is about 50% of the exhaust volume. The utility post 17 is 6 inches minimum. The depth of these prior art fume hoods were sized so they could be carried through an average door and placed on a 30″ deep by 36″ high bench with an overall height limited to the average nine and one half foot ceiling. The height and depth of the hoods made today are virtually the same size as were made sixty years ago. Fume hood depth and aisle spacing requirements tend to drive laboratory building column spacing, building size and construction cost. Narrow fume hoods cost less to manufacture and save building construction costs by allowing narrower 9-to-10 foot column spacing. Manufacturers would vary hood lengths and sash openings, but such accommodations made no functional difference.
To address rising energy costs in the early 70's, horizontal sashes were introduced to reduce the size of the sash opening. The prior art horizontal sash fume hoods used either a single track or two track configuration. The prior art lower horizontal sash panels were guided in friction channels located in the sash handle and used either rollers or a friction channel upper track as guides. The sash handle channel tracks are prone to chemical attack and collect debris, thereby preventing movement and creating turbulence as the horizontal sash is opened. Unfortunately, the prior art horizontal sash was directed toward energy savings, not worker safety. The problem with the prior art horizontal single and two track designs was that they required sash panel widths wider than workers could put their arms around to be used as a full body shield; this was a particular problem for shorter workers. Additionally, individual fume hoods are often used by two or more workers at the same time and prior art horizontal sash hoods cannot accommodate multiple workers. As a result, such prior art horizontal sash design encourages workers to work in front of an open sash with no splash or explosion protection.
The industry long operated under the erroneous assumption that the fume hood rear baffle slot adjustments were based on the fume hood's air density. The theory was to open the top slot when using lighter than air fumes and open the bottom baffle slots for heavier-than-air-fumes. Prior art patents U.S. Pat. Nos. 3,000,292; 3,218,953; 4,177,717; 4,434,711; 4,785,722; and 5,378,195 describe baffle adjustments and design based on these theories.
FIG. 3, which can be found in the 1999 American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE) engineering handbook on laboratories, illustrates the industry's perception at that time of the airflow patterns of a typical prior art face velocity capture hood to be laminar airflow. It shows laminar air 27 pattern with no vortex when vertical movable sash 18 in the raised position. In fact, U.S. Pat. Nos. 4,280,400 and 4,785,722 describe fume hood designs to eliminate vortexes from forming. Subsequent studies by Robert Morris, which resulted in several patents, provided a reversal to previously held theory that the fume hood design required eliminating or at least minimizing any vortex from forming within the fume hood. Such studies prompted ASHRAE to remove the laminar airflow FIG. 3 from their 2003 engineering handbook on Laboratories.
U.S. Pat. No. 5,697,838 to Morris taught that a fume hood effectively contained fumes when the vortex was stable and fully developed. Vortexes can be further described as developing from mono-stable to bi-stable. A mono-stable vortex is elliptical shaped and attaches to a surface as an air stream is directed across that surface. The elliptical shape is caused by a pressure gradient that forms across the vortex bubble which deforms the vortex. The mono-stable vortex has pulling and lifting forces but is restricted to amount of air volume it can sustain before it becomes unstable. A bi-stable vortex is symmetrical in shape and attaches to two or more surfaces. The bi-stable vortex has better memory and little force but can sustain a greater air volume and still remain stable. Because of cost advantages of making prior art fume hoods narrow, prior art fume hoods do not create stable vortexes throughout sash movement unless the baffle slot velocities and exhaust air volumes are automatically controlled. U.S. Pat. No. 5,924,920 to Morris et al. taught how a fume hood could be designed to form a bi-stable vortex at a full open sash and then to a mono-stable vortex as the sash is closed. One disadvantage was that fume hoods constructed according to the formula of U.S. Pat. No. 5,924,920 are required to be made deeper.
Robert Morris, inventor of U.S. Pat. Nos. 5,697,838 and 5,924,920, published studies indicate that 90% of prior art fume hood spillage appears as puffs at the sash handle which linger at the sash handle when the vortex collapses. FIG. 4A and FIG. 4B illustrate what occurs when the vortex collapses and turbulence occurs. FIG. 4A shows a containing hood with a mono-stable vortex 2. FIG. 4B shows a non-containing hood with an undefined vortex 3′, turbulence 21, and chemical spillage 4. This issue becomes a greater health risk for the less than average 5′8″ worker. Designers misinterpreting the observation of fume hood smoke pattern testing led prior art fume hood designers to focus on the face velocity and the elimination of the vortex.
In fact, however, it is during the collapse of the vortex that a hood fails to contain fumes. When the vortex fully stabilizes, the fume hood contains fume vapors. The misunderstanding of the importance of a stable vortex lead designers of prior art fume hoods to locate the introduction of bypass diffuser air above the sash handle (FIGS. 1, 3 and 4) directly into the upper vortex-forming chamber. Introduction of bypass diffuser air above the sash inhibits a stable vortex from forming within the vortex chamber and creates varying airflow patterns with sash movement.
Prior art fume hood designs are based on commonly held notions that a constant face velocity captures fumes thereby preventing spillage and should be maintained with sash window opening and closing by locating the bypass diffuser above the sash opening and controlling the exhaust airflow volume. Fume hoods based on these designs eliminate a stable vortex from forming. Additionally, prior art fume hoods baffle slots are adjusted based on fume air density, and the work surface airfoil directs air across the work surface towards bottom baffle exhaust slot. These design assumptions, as well as others, are not accurate because they fail to address the optimum airflow, and therefore the required face velocity and internal airflow patterns to prevent fume spillage through containment of the toxic fumes.